The present invention is directed to a novel heat resistant polychloroprene copolymer and process for its preparation.
Polychloroprene prepared by the polymerization of chloroprene in the presence of a mercaptan chain-terminator, often referred to as type-W neoprene, has excellent heat resistance and compression set resistance, which are characteristics that are desirable for automotive power transmission belts, but, unfortunately, such polychloroprene cannot be used in dynamic applications because the cured elastomer typically contains only monosulfide linkages. When the polychloroprene containing monosulfide linkages is subjected to flexing, the elastomer fails rapidly because of its poor dynamic properties. Polychloroprenes that are "sulfur-modified", often referred to as type-G neoprene, yield cured elastomers that are especially useful for dynamic applications, such as power transmission belts, including V-belts for under-the-hood automotive applications. The sulfur-modified polychloroprenes are prepared by polymerizing chloroprene (2-chloro-1,3-butadiene) in an aqueous emulsion system in the presence of elemental sulfur. The sulfur becomes incorporated in the polymer chain in the form of polysulfide linkages. Some of the polysulfide linkages normally are cleaved chemically after polymerization to yield polymers having viscosities suitable for processing. This cleavage step is customarily called peptization. These sulfur-modified polychloroprene elastomers are especially useful for dynamic applications such as power transmission belts and timing belts. Unfortunately, the sulfur-modified type-G neoprene elastomers have poorer heat resistance than the type-W neoprene elastomers. Additional components, such as antioxidants and antiozonants, have been added to the sulfur-modified polychloroprene to improve its heat resistance and retard the onset of hardness but further improvement by altering formulations based on presently available sulfur-modified polychloroprene elastomers does not appear likely. The increase in under-the-hood temperatures resulting from higher engine operating temperatures and front wheel drive engine configurations have heightened the need for a more heat resistant sulfur-modified polychloroprene for dynamic applications such as power transmission belts.